Continuous protection of a quantum state from motional dephasing

  Ran Finkelstein  ,  Ohr Lahad  ,  Omri Davidson  ,  Itsik Cohen  ,  Eilon Poem  ,  Ofer Firstenberg  
Physics of complex systems, Weizmann Institute of Science

Room-temperature atomic vapors are known for their simplicity and their potential scaling-up in quantum technologies. In spite of these benefits, laser-cooled atoms have evolved to be the prevalent systems for studying strong and coherent light-matter interactions, as the latter are unhindered by Doppler broadening.
Here we present a general method to overcome the effective decrease of both atom-photon cross-section and coherence time in vapor, and in fact in any inhomogeneously broadened atom-like system. Our method is based on the counteraction of the inhomogeneous dephasing of two coupled states, where one state has enhanced sensitivity to the source of dephasing. A far-detuned dressing field admixes a fraction of this "sensor" state into the "protected" state, yielding a velocity-insensitive state.
First, we utilize such dressing to achieve a significant increase in two-photon Raman absorption, surpassing the hot-atom absorption limit [1]. We have also applied this scheme with highly-excited Rydberg atoms, where absorption cross-section is the key parameter for effective photon-photon interactions. Second, we apply this method to cancellation of Doppler broadening in two-color transitions [2].
Finally, we apply this method to extend the lifetime of a collective excitation stored in a thermal atomic vapor. We use a fast quantum memory in a ladder-type system [3] and show that introducing a far-detuned dressing field during storage time completely cancels the inhomogeneous dephasing of the stored state. This method is continuous and maintains the state coherence in any given instance, in contrast to pulsed echo-based techniques, in which the state regains coherence at specific, predeterminate times only.
We will show how this method can be extended to realize strong photon-photon interactions using Rydberg states in thermal vapor.

[1] Ohr Lahad, Ran Finkelstein, Omri Davidson, Ohad Michel, Eilon Poem & Ofer Firstenberg, Phys. Rev. Lett. 123 173203 (2019).
[2] Ran Finkelstein , Ohr Lahad, Ohad Michel, Omri Davidson, Eilon Poem & Ofer Firstenberg, New J. Phys. 21 103024 (2019)
[3] Ran Finkelstein, Eilon Poem, Ohad Michel, Ohr Lahad, and Ofer Firstenberg. Science advances 4 (2018).